In recent years, making the capacity of secondary batteries, such as lithium secondary batteries, much higher has been sought. For the purpose of materializing making the capacity of lithium secondary batteries higher, the following have been investigated and proposed: improving active materials that are used for the electrodes of lithium secondary batteries; filling those active materials up with high densities; increasing the areas of electrodes; or making members that do not contribute to battery reactions smaller, such as turning separators into thin films. For example, techniques that prescribe positive-electrode particulate diameters, pore volumes or specific surface areas have been proposed (Patent Literature Nos. 1-4, and the like).
However, it is difficult to say that the means that have been proposed so far, including Patent Literature Nos. 1-4, are effective to batteries for applications that require the voltage flatness at the time of charging/discharging and the output improvement, and furthermore retaining the cyclic durability while maintaining these properties (the durability for 10 years or more, for instance), though they are effective from the viewpoint of capacity enlargement.
For example, as one of the reasons that it is difficult to expand lithium secondary batteries to the applications of electric tools and hybrid cars in which large-current charge/discharge is needed, it is possible to name their durability under service conditions entailing large-current discharge is not sufficient, compared with that of nicad batteries and nickel-hydrogen batteries.
For the purpose of solving this durability, maintaining the conductivity between positive- and/or negative-electrode active-material particles, and the conductivity between those active materials and electricity collectors have been investigated. For example, in a secondary battery whose battery life is 2-4 years approximately but is practical enough for the applications to cellular phones and notebook-size personal computers, it has been proposed to add and then mix at least one of carbonaceous mesophase spherule and vapor-phase-growth carbon fibers as a conductive material at an electrode mixture-material layer in the negative electrode, for the purpose of improving the capacity and battery life and securing their safety (Patent Literature No. 5). However, it is not guaranteed that the low-resistance conductive material is dispersed homogenously within the electrode by simply adding it, and therefore it has become a cause of the occurrence of characteristic fluctuations between cells.
Here, inmost cases of the aforementioned applications, multi cells are connected in series and are then used under high voltages, the properties, such as the capacities, outputs and resistances, fluctuate due to the degradation of the respective batteries (or cells) so that the performance lowering and drawbacks of power source itself have come to be brought about, and thereby even a drawback that occurs in a single battery has come to have a great influence on the entire power source.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-158,005;
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-236,808;
Patent Literature No. 3: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-236,809;
Patent Literature No. 4: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-89,118;
Patent Literature No. 5: Japanese Patent Gazette No. 3,867,030; and
Patent Literature No. 6: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2006-244,984